Typically, the fillers or pigments that are used in the manufacture of paper and board have an average diameter of less than 5 μm and have a light colour. The most typical fillers include kaolins, talcs, ground calcium carbonate (GCC), and precipitated calcium carbonate (PCC). In addition, there are more expensive special pigments, such as precipitated aluminium silicates, satin white, and titanium dioxide. The drawing of an exact line between fillers and coating pigments is difficult; however, roughly speaking, fillers have a coarser size than pigments, which are used in coating. From the point of view of maximum light scattering, in theory, an optimal particle size for the most common fillers and coating pigments is about 0.4-0.5 μm. Typically, the average particle size of coating pigments is 0.5-1 μm and that of fillers is 1.5-4 μm. In this application, these concepts are, hence, not distinguished, but pigments and fillers are jointly called the filler. In the manufacture of paper and board, fillers are preferably used, because they replace more expensive fibres, and improve the optical properties and the setting of printing ink (the printability). Using them, the basis weight of paper or board can also be reduced, whereby the same weight provides an increased printing or packing surface without the quality suffering.
The greatest disadvantage of using fillers and pigments, as well as other fillers, is the weakening of the dry strength of the paper or board structure, when the chemical mass is replaced with filler, in particular. This is due to the fact that fillers prevent the formation of hydrogen bonds between the fibres, by attaching themselves to the surface of the fibres. Calcium carbonates, both ground (GCC) and precipitated (PCC), however, are widely used, because of their low price and good light scattering properties, in replacing chemical pulp fibre, in particular. Certainly, paper and board, which are manufactured from recycled, de-inked and mechanical pulps, could be replaced with calcium carbonates, but the alkali darkening of mechanical pulp often limits their use in the manufacture of the paper and board grades that are manufactured from these pulps. The decrease in strength and stiffness of the paper or board product, when the fibre is replaced with filler, is mainly due to the fact that fillers weaken the formation of hydrogen bonds between the fibres, since the surface of the fillers does not form hydrogen bonds. In the same basis weight, an increase in the filler content causes an increase in the density of the paper and a decrease in the thickness of the paper. The latter causes a decrease in the stiffness of the paper or board. At present, filler is normally added directly to the pulp. In the wire section, only part of the added filler is attached to the finished paper or board web. The reminder of the filler travels through the white water system to finally constitute part of the paper or board structure. Part of the filler of the white water system, finally, also burdens the sewage treatment plant, since not all of the material is ever carried out of the process along with the finished paper or board. The weaker fibre-fibre bonds that are caused by the filler on the surface of the paper or board may also result in an increase in the dust formation of the surface in printing, when the surface strength weakens.
Precipitated calcium carbonates (PCC) can be manufactured in separate factories, whereby the finished PCC is delivered to the customer as slurry or a dry product. At the present time, however, PCC factories are generally built close to paper or board mills, whereby PCC is delivered in a form of slurry, through a pipe, to the storage container of the customer. One advantage of these “on-site factories” is that, in the precipitation of PCC, the carbon dioxide that is released in the manufacturing process of chemical pulp can then be exploited.
It is typical of these precipitation processes of PCC that the pH is reduced from the alkaline range to the neutral one (typically, pH of 7-8.5), by decreasing the pH of an alkaline Ca(OH)2 solution by an acidic CO2. In recent years, manufacturing processes have been launched on the market, precipitating PCC directly to the fibre slush at the paper or board mill. These precipitation processes often employ an intensive agitation or injection pressure when admixing Ca(OH)2 and CO2 to the fibres. In this case, the precipitation pH typically lowers from alkaline to the neutral range (pH 7-8.5), or it is kept essentially neutral by changing the mutual dosages of Ca(OH)2 and CO2. Thus, it is common to the processes mentioned above that the pH in the carbonate solution is alkaline before the precipitation, whereby they are not suitable to be used as part of the manufacture of paper or board.
Dry strength is a structural property of paper or board, which mainly develops when the dry matter content of a wet fibre network increases, when water is removed from the fibre slush by filtering, pressing, and drying. The strength of the finished paper and board consists in the strength of single fibres, the bonds between the fibres, the number of the bonds, and the distribution of the bonds and fibres in the fibre network. The distribution is essentially influenced by formation.
Various forces influence the strength of the fibre network; the most important of these comprises hydrogen bonds, even though covalent bonds, ionic bonds, and van der Waals forces have a specific effect on the strength of the network. The number of hydrogen bonds is large, and they act close to the surfaces of the fibres.
Dry strength agents enhance the strength of other properties in the network, but they do not influence the strength of single fibres. It is a known fact to also mechanically increase the strength of the fibre network by grinding the fibres in water. In that case, the number of micro-fibrils on the fibre surfaces grows, increasing the number of fibre bonds and distributing the fibres more evenly.
The strength of the paper or board can be increased by increasing the portion of long fibre in the fibre composition, decreasing the amount of filler that is used, or adding a dry strength agent to the fibre slush. The process changes that can be used to strengthen the fibre network include raising the pH of the wet section (from acidic to neutral), an improved formation, and a stronger wet pressing in the press section.
Grinding the fibre slush, however, is the most typical way of increasing the dry strength of paper or board. Apart from an increase in the energy consumption, the negative effects of grinding include an increase in the density of the finished paper or board, and a decrease in the porosity, stiffness, and tearing strength. Typically, the opacity also decreases along with the grinding of fibre slush.
Due to these adverse effects of the fibre slush grinding, it is normal to use dry strength agents. Typically, the dry strength agents are water-soluble, hydrophilic polymers that are either natural or synthetic products. The best commercial products comprise starch, vegetable gums, carboxy-methyl cellulose, and, regarding synthetic polymers, polyacrylamide and glyoxylated polyacrylamide, in particular.
All plants contain starch. Starch is commercially produced from potato, tapioca, barley, wheat, rice, and corn, however. Waxy maize is a starch that is refined in the United States, fully consisting of amylopectin. Waxy maize is used instead of potato starch, in particular.
Starch belongs to polysaccharides. Starch is a glucose polymer, wherein the anhydroglucose units are bonded to each other by a 1,4-α-D-glucosidic bond. The glucose chains are either straight (amylose) or branched (amylopectin). Normally, the amount of amylose in starch is smaller. In plants, starch is found in small (2-150 μm) granules that are separated in the manufacturing process.
The ability of starch to attach to the fibre network is caused by its large number of hydroxyl groups, which increase its ability to form hydrogen bonds. The hydroxyl groups also bind hydrogen molecules to themselves. In the drying of paper or board, water evaporates and hydrogen bonds are formed between starch and the fibres.
Generally, starch does not dissolve in cold water. This is due to the fact that the starch polymers are in a well-organized form, bound by the hydrogen bonds in the starch granules. When an aqueous starch solution is heated, the starch granules swell first, after which, single starch polymers are released from each other.
Starch is often treated to make the starch cationic, the stability of the solution is increased and/or its rheological properties are improved at higher dry matter contents. Being a polyol, the chemical treatment products of starch are generally ethers or esters. These treatment alternatives include hydroxy-alkylation, cationization, carboxymethylation, acetylation, thermo-mechanical treatment, enzyme treatment, hydrogen peroxide treatment, sodium hypochlorite treatment, and acid treatment. Starches that dissolve in cold water, so-called cold-soluble starches, can also be provided by treating the starch.
After fibres and fillers, starch is a raw material that is used the most in the manufacture of paper and board. In addition to the improvement of dry strength in the paper and board manufacture, starch is used as a retention agent, for the dispersion of stock sizes, in fixatives, as a spray starch, in surface sizing, and in coating.
Native (untreated) starch is anionic and, therefore, its attachment to the fibre in the paper and board manufacturing process without a cationic treatment is poor. At present, cationic starches are ethers that are manufactured using an epoxy chemical that contains a quaternary ammonium group. The cationization considerably improves the attachment of starch polymers to the fibres. Like cationization, other treatments of starch can also be carried out for dry starch granules or starch granule slurry.
In the paper and board manufacturing process, typically, about one third of the starch is adsorbed on the surface of long fibres. This is equivalent to about 70% of the total solid matter content in the fibre, the remainder being divided evenly between the fines and the filler. A high adsorption of starch on the fines and filler results in a weakened strength. The best effect of the increase in strength of starch is achieved, when it attaches to the long fibres. In the paper and board manufacturing process, this is generally the purpose of dosing starch into high consistency pulp. When dosed into dilute pulp, close to the head box, starch (mainly cationic) improves the retention and dewatering—not so much the strength. The use of starch in surface sizing does not result in an equally effective improvement of strength as when added to high consistency pulp.
The main purpose of pulp starch is to improve the dry strength of the paper or board. Starch improves the strength and tensile strength of the paper and board in the Z direction, in particular. The use of pulp starch also considerably improves the bursting strength that is of importance to the manufacturers of board and corrugated board. When starch is added to high consistency pulp to increase the strength, normally, 4-10 kg/t of starch is used. Generally, increasing the amount of addition does not considerably increase the strength properties, but weakens the dewatering of the wire section, among others, even though as much as 40 kg/t of additions have been reported. This is due to the fact that when the anionic charge of the fibres and the filler is neutralized with cationic starch, the rest of the starch no longer attaches to the fibres and the filler, but remains as dissolved starch in the circulation waters.
Typically, the starch polymers inside the starch granules should either be released by adding them to hot water or cooking the aqueous starch solution. An exception to this is constituted by the cold-soluble starches mentioned above. At present, a continuous jet cooker is the most typical method of cooking the starch. In the jet cooker, starch slurry is pumped through the cooker, while the cooker is heated with high-pressure steam. The maximum dry matter content of the starch solution in the jet cooker can be over 10% and the temperature is about 125-135° C. The cooking time is 2-4 minutes. A suitable storage temperature for cooked starch is 60-80° C. In batch cooking, starch is typically heated in a 5% solution to 95° C. with direct steam, and the cooking time is 20-30 minutes. When dosed into the paper or board machine, starch is diluted to 1% or weaker.
In addition to starch, other strength-improving additives have also been used. Guar gum is a vegetable gum that is used the most frequently. Guar gum and locust bean gum are seed gums and consist of galactomannan. Karaya gum has a more complex structure; it is a branched polysaccharide. The use of vegetable gums has been limited by their price that is higher than starch. They have also been more difficult to treat than starch. The additives that improve the dry strength, which are used the most, comprise cationic starches. The anionic charge of vegetable gums has enabled them to be used, to a minor degree, to prevent over-flocculation, improving the formation. Obviously, they then act as some kind of a protective colloid between the pulp components.
Carboxy-methyl cellulose (CMC) is water-soluble cellulose that is treated by carboxylation. The carboxyl content and the length of the molecular chain vary between different products. CMC improves the dry strength of paper and board similarly to starch, but its use in the applications of the wet section has been limited to special papers, mostly because of its high price. The carboxyl groups of CMC render it anionic; therefore, when used in the wet section, another cationic additive should be used to attach the same.
Cellulose and nanocellulose also belong to polysaccharides. Nanocellulose or microfibrillated cellulose can be manufactured from all materials that contain cellulose, such as wood. The structure of nanocellulose is considerably smaller than that of a normal cellulose polymer; therefore, it contains a considerably larger number of hydroxyl groups that form hydrogen bonds. Cellulose that has been pre-treated with enzymes or carboxymethylation makes the manufacture of nanocellulose cheaper. At present, the manufacturing process requires high pressure and temperature, and high speed in the homogenizer; without the pre-treatment, the manufacturing costs are considerably higher.
Polyacrylamide (PAM) is the most frequently used synthetic polymer, which is used to improve the dry strength. PAM is long and straight-chain and it either has a cationic or anionic charge. Due to its high price, PAM is generally used as a retention agent, instead of trying to improve the dry strength. For the improvement of the dry strength of polyacrylamide, glyoxylated PAMs are also found on the market. Regarding other synthetic polymers, which can be used to improve the dry strength, polyvinyl alcohol and latex should be mentioned.
Historically, efforts have been made to solve the problem of reduced strength, which is caused by the addition of filler, by agglomerating single filler particles into larger agglomerates. Such patents include, among others, the U.S. Pat. Nos. 4,225,383; 4,115,187; 4,445,970; 5,514,212; and 4,710,270; and the GB patents 2,016,498 and 1,505,641. In these patents, anionic filler particles have typically been agglomerated into larger aggregates with a cationic additive in a mixture. In these patents, it has been observed that the strength properties and filler retention have improved, but at the same time, the optical properties have decreased.
There are also patents, wherein latexes are utilized to reduce the decrease in strength that is caused by the addition of fillers. Such patents include, among others, the U.S. Pat. Nos. 4,178,205; 4,189,345; 4,187,142; 4,710,270; and 7,074,845B2. The U.S. Pat. No. 4,799,964 and the U.S. Patent application Publication No. 20020100564 deal with the manufacture of a filler agglomerate, using starch as a binder. They prevent the decrease in strength that is due to the use of filler, but do not increase the strength, even though filler is used.
The U.S. Patent application Publication No. 20080087396 relates to a filler that is coated with starch, resulting in a lower decrease in strength than when untreated filler is used. The US Patent Publication Nos. 20100179248, 20050252629, 20030188738, 20100181038, 20100181037 and 20100078138, as well as the U.S. Pat. No. 8,025,768, respectively, deal with the treatment of filler with starch and/or latex to obtain a higher filler content in the paper or board, while the strength is better maintained. The end products obtained from these treatments are also filler agglomerates, as above.
The U.S. Patent applications Publication Nos. 20070101904, 20090255441, and 20070163737 relate to an organic filler or pigment, which is manufactured from starch and which does not reduce the strength of the manufactured paper or board at all, because organic starch strengthens the fibre network, contrary to the inorganic fillers and pigments that are commonly used. One challenge thereof, again, is the price that is higher than that of the fillers, which fully or partly consist of inorganic substances.
Consequently, there is a need for a fibre product, wherein the filler would attach to the fibre and fillers more effectively and, at the same time, would give the product advantageous strength properties that would preferably be further improved, compared to the known solutions.